This work was initiated to study the role of cell interactions in the development of syngeneic tumor immunity. It has been shown that the degree of specific anti-tumor immunity developed both in vivo and in vitro in response to TNP-conjugated tumor cells can be increased by the addition to TNP-reactive helper T cells. The cytotoxic cell in vitro has been shown to be a T-cell. The nature of the lymphocytes involved in in vivo killing is under investigation.